The present invention relates to filters, particularly filters which include cavity resonators, for use in microelectronic devices and assemblies, e.g., chips, substrates and circuit panels.
Filters play a critical role in the operation of radio receivers and transmitters. In receivers, high-Q filters are used to confine received signal energy to narrow passbands in order to reject noise and spurious harmonics that interfere with the reception of the intended signal. In transmitters, high-Q filters are used to restrict the bandwidth of signals to be amplified to designated channels, for example, for the purpose of increasing the signal to noise ratio of the transmitted signal and to avoid the transmitted signals from interfering with out-of-band signals.
Many filters used in microelectronics employ lumped components, e.g., capacitors and inductors, which are combined to form resonant circuits, for example, to select a narrow fixed passband of an intermediate frequency (“IF”) or baseband (“BB”) signal in a receiver. Lumped components may be provided as discrete components mounted to a circuit panel or other interconnection element. Alternatively, distributed components or both lumped and distributed components may be provided as elements of a chip or microelectronic substrate commonly known as “integrated passives on chip” (IPOC). The Q value of each component in a filter strongly influences the overall performance of the filter. In order for filters to provide good rejection of out-of-band energy and noise, they need to operate with a high “Q” value. High Q values generally result in the following benefits: a greater degree of signal isolation, the ability to achieve narrower passbands, and sharper filter roll-off.
Unfortunately, the Q value of traditional lumped components is inadequate for these purposes. Traditional lumped capacitors used in microelectronic devices, such as in capacitors of an IPOC, typically have unloaded Q values which are below 200. Lumped inductors used in microelectronics, e.g., inductors which are formed as traces on a microelectronic dielectric sheet, typically have unloaded Q values which are below 100. Such unloaded component Q values lead to circuit Q values of about 10 in resonant circuits which include the components. Circuit Q values of about 10 are inadequate to achieve the above-indicated goals.
Outside the field of microelectronics, one class of resonant circuits, the helical cavity resonator, has a characteristically high unloaded Q value. Such resonators typically have Q values ranging between about 500 and 1000 over frequencies selected between about 10 MHz and 1000 MHz. As used herein, a “cavity resonator” is defined as a chamber, which may be hollow, or, alternatively packed with a dielectric material, whose dimensions allow the resonant oscillation of electromagnetic waves, and in which is disposed an inductive element for exciting the electromagnetic waves.
An example of a two-stage helical cavity resonator 50 is illustrated schematically in a sectional view thereof in FIG. 1. As shown therein, the helical cavity resonator 50 includes a shielded enclosure 60, which encloses a first volume 62 and a second volume 64 and having an internal shield 66 which separates the first volume 62 from the second volume 64. The shielded enclosure 60 is cubic or cuboid in shape, having planar walls 61, each of which presents a conductive surface to the first and/or second volumes 62, 64 enclosed by the shielded enclosure. The shielded enclosure 60 is typically held at a fixed potential, e.g., ground, thus forcing the conductive interior surfaces of the walls 61 of the enclosure to be held at that potential, e.g., ground.
A first helical coiled inductor 70 is disposed within the first volume 62 and a second helical coiled inductor 72 is disposed within the second volume 64. The first inductor 70 has a ground end 71 mounted to the shielded enclosure 60, shown here as being mounted to the internal shield 66. Likewise, the second inductor 72 also has a ground end 73 mounted to the shielded enclosure 60, also shown as being mounted to the internal shield 66. In addition, the first inductor has an open end 74 and the second inductor has an open end 76. Connected to the first inductor 70 is a first transmission line 80 having a characteristic impedance such as 50Ω. A second transmission line 82 is connected to the second inductor 72, also having a characteristic impedance which is typically the same as that of the first transmission line 80, e.g., an impedance of 50Ω. Each of the first and second transmission lines extends from inside the shielded enclosure 60 through openings 86, 88, respectively, to the space 85 outside the shielded enclosure. Transmission line 80 includes an active conductor 87 and a grounded conductor 81. Transmission line 82 includes an active conductor 89 and a grounded conductor 83. The grounded conductors 81, 83 typically are in conductive communication with the shielded enclosure 60, and/or one or more external ground points (not shown) in order to provide a stable ground for the transmission lines.
One requirement in fabricating the helical cavity resonator is to attach the active conductors 87, 89 of the transmission lines to the inductive elements 70, 72, respectively, at a location which terminates the transmission lines in matched impedances. Unfortunately, achieving such terminations is difficult. Because the cavity resonator is very sensitive to variations in dimensions and the shape of the inductive element, painstaking manual adjustments must be made in order to achieve the matched impedance. Moreover, the same hand-tuning must be performed for each such cavity resonator being manufactured, because the dimensions and shape of the inductor (and hence, its impedance) are subject to variations.
The helical cavity resonator 50 operates to resonate at a predetermined resonant frequency f0 which is determined by the inductance of the inductors 70, 72 and the dimensions and geometry of the shielded enclosure 60. Due to the boundary conditions imposed by the walls 61 of the shielded enclosure, an electromagnetic field of standing waves is excited in the first volume at the resonant frequency f0. Such electromagnetic field is excited by an excitation current delivered onto the first inductor 70 by the first transmission line 80. An opening 68 is provided in the internal shield 66 for the purpose of coupling energy from the electromagnetic field excited by the first inductor 70 onto the second inductor 72. The first and second inductors 70, 72 and the shielded enclosure 60 cooperate together in exciting a current in the second inductor 72 having an amplitude which is very sensitive to the frequency of the excitation current present on the first inductor 70. The excited current on inductor 72 is output onto the second transmission line 82. The excited current output onto transmission line 82 is the same as or exceeds the excitation current provided onto inductor 70 when the frequency of the excitation current is at the resonant frequency of the cavity resonator 50. However, very little excited current is produced in the second inductor 72 unless the frequency of the excitation current is at or near the resonant frequency. In this manner, the helical cavity resonator 50 operates as a filter to select a narrow passband between a signal arriving on first transmission line 80 and output onto second transmission line 82.
The merits of the helical cavity resonator are best illustrated with reference to FIG. 2. As shown therein, FIG. 2 is a chart comparing the frequency response of a first type of bandpass filter which employs a helical cavity resonator (curve 10), as compared to a second type of bandpass filter (curve 12) which employs lumped circuit elements. In addition, the first filter type has a much narrower passband. Each of the first and second filter types has a nominal center frequency f0. By convention, the passband is generally considered to be the range of frequencies which lie between a lower “3 dB frequency” and an upper “3 dB frequency”. The lower 3 dB frequency is the frequency below f0 at which the frequency response is 3 dB lower than the peak frequency response (the frequency response at f0). The upper “3 dB frequency” is a frequency above f0 at which the frequency response is 3 dB lower than the peak frequency response. As illustrated in FIG. 2, the passband 20 of the first filter type lies between the lower 3 dB frequency 30 and the upper 3 dB frequency 32. On the other hand, the passband 22 of the second filter type lies between the lower 3 dB frequency 40 and the upper 3 dB frequency 42. As apparent from FIG. 2, the passband 20 of the first filter type, having a helical cavity resonator, is much narrower than the passband 22 of the second filter type having lumped circuit elements.
Unfortunately, the available helical cavity resonators available heretofore are heavy, expensive and bulky, typically being constructed of helical coils of copper tubing which is disposed within in metal chambers. Aside from that, fabrication of such resonators is difficult. In particular, the task of properly terminating the helical inductor element in such resonators is expensive and arduous, because the 50 ohm termination point is difficult to determine prior to constructing the helical coil and the metal chamber. Because of the size and weight of helical cavity resonators and the difficulties involved in providing the appropriate termination point, heretofore the use of such resonators has been limited to applications outside the field of microelectronic devices and microelectronic assemblies.
However, as explained above, there is a present need for resonant circuits in microelectronics having higher Q values. Accordingly, it would be desirable to provide a new high Q value resonator component suitable for use in or with microelectronic assemblies.